1. Field of the Invention
This invention relates generally to an overvoltage protection circuit for a pair of telephone lines and more particularly, to circuitry for filtering noise and transient voltage signals above data signals on a pair of telephone lines.
2. Prior Art
Various arrangements have been used for protecting telephone equipment against hazardous voltages due to lightning or power surges. However, conventional arrangements are effective for clamping transient voltages at a rated potential above the ring signal of conventional telephone systems. The typical ring signal is about 170 volts AC peak with a frequency between 15 and 30 hertz so that the conventional arrangements have a rated clamping voltage of approximately 200 volts. Consequently, such arrangements are ineffective for filtering noise and transient voltage signals occurring below this standard clamping voltage.
Overvoltage protection circuitry that overcomes many of the disadvantages of the prior art is disclosed in McCartney, U.S. Pat. No. 4,758,920, issued July 19, 1988 and assigned to the same assignee of the present invention.
Known protection circuitry generally effective for normal mode overvoltage noise and transient protection can produce a differential current flow in the lines in response to common mode induced voltages of the same phase and amplitude on both lines. Such differential current flow can result from a common mode dynamic unbalance in the protection circuitry and cause an audible hum. It is desirable to provide an overvoltage protection circuitry that avoids introducing any common mode dynamic unbalance differential current flow in the lines in response to common mode induced voltages or audible hum.